Thermopower measurements on pentacene transistors

We present thermoelectric measurements on pentacene field-effect transistors. We report high values of the Seebeck coefficient at room temperature between 240 and 500 mu V/K depending on the dielectric surface treatment. These values are independent of the thickness of the channel and of the applied gate voltage. Two contributions to the Seebeck coefficient are clearly identified: the expected contribution that is dependent on the position of the transport level and reflects the activated character of carrier generation and an unexpected intrinsic contribution of 265 +/- 40 mu V/K that is independent of the temperature and the treatment of the oxide surface. This value corresponds to an unusually large lattice vibrational entropy of 3k(B) per carrier. We demonstrate that this intrinsic vibrational entropy arises from lattice hardening induced by the presence of the charge carrier. Our results provide direct evidence of the importance of electronic polarization effects on charge transport in organic molecular semiconductors.